1. Field of the Invention
The invention relates generally to the field of determining position of a wellbore within subsurface Earth formations. More specifically, the invention relates to methods for using azimuthally dependent formation property measurements of such formation to more precisely locate the position of the wellbore and to refine models of the Earth's subsurface structure.
2. Background Art
Well bores are drilled through subsurface Earth formations to extract useful materials such as oil and gas that are present in certain subsurface formations. Wellbore drilling is typically performed in a manner to optimize the amount of oil and gas bearing formation that is in hydraulic communication with a wellbore. Such positioning increases the effective drainage radius of the wellbore within the producing formation.
In order to optimize such wellbore drilling, it is known in the art to produce a model of the structure of the Earth's subsurface formations. Such models may be initially generated using techniques that do not use data from within the subsurface formations, such as seismic surveying and electromagnetic surveying. Such surveys are interpreted to produce an initial estimate or model of the spatial distribution of the subsurface formations, including those that may contain oil and/or gas.
As one or more wellbores are drilled through the formations that have been modeled using the above seismic and/or electromagnetic surveying techniques, the model may be adjusted or updated to reflect information obtained during or after the drilling of such wellbore(s). Such information is generally obtained in the form of “well logs,” such well logs being a record with respect to position along the wellbore of various physical parameters. Such parameters may include, for example, electrical conductivity (resistivity), acoustic velocity, density, neutron porosity and natural gamma radiation and formation fluid pressure among others. Such well logs may be made during the drilling of the wellbore, using so called “logging while drilling” (LWD) measurements, or afterward, using well logging instruments conveyed along the wellbore using armored electrical cable or other known conveyance technique. A model may also be initially generated based on well logs alone.
In order to adjust or update the initial model of the Earth's subsurface structure in a useful manner based on such well log data, it is necessary to know with reasonable precision the geodetic position of the wellbore at every point along its length, and the precise position along the wellbore of the particular well logging instrument whose measurements are used to adjust the model. The position along the wellbore is referred to as the “measured depth” and may be reasonably precisely determined using techniques well known in the art.
Determining the geodetic position of the wellbore at any point along its length is typically performed using directional sensors disposed in the well logging instrument. Such directional sensors may include magnetometers to determine wellbore direction with respect to the Earth's magnetic poles and accelerometers to determined the inclination of the wellbore from vertical (gravity). It is also known in the art to use inertial navigation devices to determine geodetic direction of the wellbore. Irrespective of the type of directional measurement instrument being used, limits on their accuracy and precision result in some degree of uncertainty as to the absolute geodetic position of the wellbore. There are corresponding limits to the accuracy and precision of the initial models made from seismic and/or electromagnetic surveys. As a result, in some wellbore drilling operations, wherein it is desirable to maintain the wellbore trajectory within a particular formation within the Earth's subsurface, the degree of uncertainty as to the relative positions of the wellbore and the subsurface formations may limit the ability of the wellbore operator to so maintain the wellbore trajectory.
More recently, apparatus and methods have been developed that provide formation property measurements that are directionally (azimuthally) sensitive, and can provide estimates of the distance from the well logging instrument to one or more formation boundaries (wherein a mineral composition and/or fluid content of the formation changes). See, for example, U.S. patent application Publication No. 2005/0140373 filed by Li et al. and assigned to the assignee of the present invention.
There continues to be a need to more precisely determine the position of a wellbore within structures in the Earth's subsurface and to be able to navigate wellbores during drilling to maintain such position along a desired trajectory with respect to subsurface formations.